The present invention relates to a novel process for the purification of crude aromatic carboxylic acids, such as, for example, terephthalic acid, orthophthalic acid, trimesic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid and pyromellitic acid, which are obtained by oxidising the corresponding aromatic precursors.
Processes for the production of aromatic mono- and poly-carboxylic acids are well known in the literature; they are normally carried out in liquid phase, using as substrates aromatic compounds having at least one oxidisable substituent group which is attached directly to the carbon atom of the corresponding aromatic nucleus (where the expression xe2x80x9coxidisable substituent groupxe2x80x9d is intended to indicate any substituent in which a carbon atom is bonded directly to the aromatic nucleus) and which, as a result of oxidation, is converted into a carboxylic group.
The oxidising agent is generally gaseous molecular oxygen, preferably diluted with an inert gas; for obvious practical reasons, air (optionally enriched with molecular oxygen) is the gaseous mixture most commonly used for the purpose; the reactions are carried out in the presence of a catalytic complex composed of one or more metals, normally in the form of salts soluble in the reaction solvent, generally acetic acid, preferably in the presence of an activator.
The processes in question are described, for example, in GB-1063964, U.S. Pat. No. 5,112,592, EP-475926, WO 98/29378, EP-26507 and EP-641597, which are to be regarded as an integral part of the present description.
However, the aromatic acids obtained in accordance with the above-mentioned processes contain a variable amount of impurities, generally aldehydes, originating from the partial oxidation of the corresponding aromatic precursor; typical impurities are, for example, 4-carboxybenzaldehyde (4-CBA) in the case of the production of terephthalic acid, and 3-carboxybenzaldehyde (3-CBA) in the case of isophthalic acid.
The purification of the aromatic mono- and poly-carboxylic acids is normally carried out by subjecting the crude product in aqueous solution, generally a 20 to 25% by weight solution, to a hydrogenation reaction in heterogeneous phase in the presence of suitable catalysts in order to convert the above-mentioned impurities into derivatives that are easier to separate from the desired end product; the hydrogenation reaction is normally carried out at pressures of approximately from 65 to 75 bar and at temperatures of approximately from 250 to 300xc2x0 C. Processes of that type are described, for example, in United States patents U.S. Pat. No. 3,522,298; U.S. Pat. No. 3,542,863; U.S. Pat. No. 3,584,039; U.S. Pat. No. 3,591,629; U.S. Pat. No. 3,607,921; U.S. Pat. No. 3,726,915; U.S. Pat. No. 3,799,976; U.S. Pat. No. 4,126,638, U.S. Pat. No. 4,260,817, U.S. Pat. No. 4,467,110, U.S. Pat. No. 4,629,715, U.S. Pat. No. 4,892,972, U.S. Pat. No. 5,362,908 and U.S. Pat. No. 5,616,792.